feat(plex): honest transcode cold-start + encoder in stats (E6 S1d)

Surface the transcode session's runtime facts in the player. The session
response's transcode/hw flags now thread through: the stats overlay's mode row
reads "transcode · GPU" (VAAPI) or "transcode · CPU" (libx264) — the at-a-glance
confirmation of which S1 path engaged — and the initial-load overlay says
"Preparing stream…" instead of "Loading…" for a transcode session, since its
first segment is genuinely slower than a remux and shouldn't read as a hang.

fmtMode (pure, unit-tested) appends the encoder only for a transcode with a known
encoder. i18n HU/EN for the new preparing label. Browser-UAT'd on the XviD
fixture: mode row reads "transcode · CPU" live (software, no render node here).

frontend/src/lib/plexStats.test.ts

@@ -7,6 +7,7 @@ import {
   fmtCodec,
   fmtFrames,
   fmtLevel,
+  fmtMode,
   fmtRes,
   type PlexStatsInput,
 } from "./plexStats";
@@ -110,7 +111,25 @@ describe("buildStatRows", () => {
     expect(rows.find((r) => r.key === "position")?.value).toBe("2:10 / 86:54");
   });
 
+  it("appends the transcode encoder to the mode row when known", () => {
+    const gpu = buildStatRows({ ...base, playable: "transcode", hwEncoder: true });
+    expect(gpu.find((r) => r.key === "mode")?.value).toBe("transcode · GPU");
+    const cpu = buildStatRows({ ...base, playable: "transcode", hwEncoder: false });
+    expect(cpu.find((r) => r.key === "mode")?.value).toBe("transcode · CPU");
+  });
+
   it("shows an empty classification honestly as a dash", () => {
     expect(buildStatRows({ ...base, playable: "" }).find((r) => r.key === "mode")?.value).toBe(DASH);
   });
 });
+
+describe("fmtMode", () => {
+  it("appends the encoder only for a transcode with a known encoder", () => {
+    expect(fmtMode("transcode", true)).toBe("transcode · GPU");
+    expect(fmtMode("transcode", false)).toBe("transcode · CPU");
+    expect(fmtMode("transcode", null)).toBe("transcode"); // encoder unknown → bare mode
+    expect(fmtMode("transcode")).toBe("transcode");
+    expect(fmtMode("remux", true)).toBe("remux"); // not a transcode → no encoder suffix
+    expect(fmtMode("")).toBe(DASH);
+  });
+});